Methods and apparatus have heretofore been devised for making liquid soap in relatively small quantities for domestic use. Exemplary of such prior art methods and apparatus are those disclosed in U.S. Pat. Nos. 64,099, 2,800,398, 2,876,082 and 2,879,143. These devices have included means for supplying animal or vegetable tallow or fat and a caustic alkali in measured quantities into a heated mixing chamber into which water is also fed. Some of the apparatus also have means for transferring liquid soap to a dispenser following saponification.
The prior art methods and apparatus apparently have not met with substantial commercial success due to inconsistency of the quality of the liquid soap that is produced by their use. To make good liquid soap in an automated, timed process where a mixture of fat, caustic and water is agitated in a heated vessel so that the soap has consistently reliable and homogeneous viscosity, texture and feel, the process would require that substantial complete saponification occur between the soap making ingredients. However, in making liquid soap in a mechanical, timed process in the typical home it is anticipated that the various animal and vegetable fats that are available in the home will be used, often even mixed together, and added to the other ingredients. If a single type and quantity of caustic is used to react with a variety of fats and combinations of fats in varying volumes, a set time for mixing the ingredients does not always coincide with the period required for proper saponification, and an automated, timed process does not always function to provide a desirable product. In other words, some fats react with a particular alkali of a particular strength in so much time while others take a shorter or longer period to saponify. Too short a mixing period will mean that some of the ingredients will not have had enough time in which to react, thereby leaving fat in the product which produces a greasy feel. Conversely, too long a mixing period is not energy efficient and can cause, with a measured quantity of diluent, the viscosity of the soap to become excessive.
Compounding the problem is the fact that when a soap mixture is being agitated to cause saponification, there are likely to be several viscosity changes throughout the mixing process, and the use of viscosity sensors alone is not reliable to determine proper saponification of the mixture. Also, if agitating the mixture under elevated temperature is suspended prior to proper saponification, further saponification is likely to occur while the liquid soap is in storage, thereby causing the viscosity of the soap to become excessive. The problem is even further compounded by the use of too much fat in comparison to too little alkali and vice versa. These problems have provided a severe restriction on the acceptability of those methods and apparatuses for producing soap except where such is done on a mass commercial basis as is carried out in commercial factories. The present invention therefore is directed at overcoming limitations and restrictions of the prior art methods and apparatuses for producing liquid soap in limited quantities in the home.